1. Field of the Invention
The present invention relates to a porous hollow fiber which is suitable for separation of substances contained in a fluid, particularly water, for the purpose of purification thereof. More particularly, this invention relates to a porous hollow fiber which is prominently improved in hydrophilic nature and has a function of filtration in a wide range such as micro-filtration and ultrafiltration.
2. Description of Prior Art
A porous hollow fiber of a polymer material is used widely as a device for precise filtration and ultrafiltration of various substances. For example, it is used for separation and purification of water or aqueous solutions in arts of industrial fields and the clinical field.
In the case that the precise filtration or ultrafiltration of water or aqueous solutions is performed by using a porous hollow fiber or film, the porous hollow fiber or film needs to have hydrophilic nature. As a process of producing a porous hollow fiber or porous film having the hydrophilic nature, there is known a process which comprises the steps of dissolving a polymer material in an appropriate solvent mixture containing a volatile solvent and a swelling agent or poor solvent to prepare a homogeneous solution, casting the solution to form a film, then removing the volatile solvent partially or completely through evaporation, followed by further removing through extraction of the remaining non-solvent or swelling agent by immersing the film in a solidifying medium to form a large number of micropores in the film. This process is called, a phase transition process. Also known is a process comprising steps of preparing a porous film of a polymer material containing a soluble material dispersed therein and then removing said soluble material by dissolving it with an appropriate solvent to form a large number of micropores in the film. The above-described processes comprising removing the soluble material are referred to hereinafter as Conventional Process 1.
Recently, an alternative process using a hollow fiber or film of a thermoplastic resin such as high-density polyethylene, polypropylene and poly(4-methylpentene-1) has been proposed and now is used in practice. In this process, the hollow fiber or film which has been spun is heat-treated and then subjected to a drawing process at a temperature of specific range and/or in a specific medium to form pores in the surrounding wall of the fiber or in the film. The process is referred to hereinafter as Conventional Process 2. In this process, a porous hollow fiber or film having excellent mechanical properties can be obtained.
However, there are drawbacks in the above-mentioned conventional processes. For instance, the process for preparing a hollow fiber or film having hydrophilic nature according to Conventional Process 1 requires a complicated process as described hereinbefore, and as a result the manufacturing cost becomes high. On the other hand, since the porous hollow fiber or film prepared in Conventional Process 2 is hydrophobic, the resulting fiber or film needs to be subjected to a supplemental procedure for providing thereto hydrophilic nature by the use of alcohol or the like when the resulting fiber or film is to be used for filtration of water or aqueous solutions, and further needs to be kept in wet state. In this process, accordingly, it becomes complicated to control or maintain the resulting fiber or film under satisfactory conditions.
The present inventors studied to improve the abovedescribed conventional porous hollow fiber, and already found that a porous hollow fiber having a satisfactory hydrophilic nature for filtration of water or aqueous solutions and a function of filtration in the wide range such as precise filtration and ultrafiltration can be obtained by coating a conventional hollow fiber on the surfaces of its surrounding wall having a large number of micropores, particularly on the walls of the micropores, with ethylene-oxide grafted nylon such as nylon 6. The invention based on the above finding is described in U.S. patent application Ser. No. 870,709 which issued as U.S. Pat. No. 4,654,265.